1. Field of the Invention
The present invention relates to shaping glass sheets of soda-lime-silica composition to complicated curvatures. The term "complicated curvatures" refers to shapes having portions of concave elevation and other portions of convex elevation in a continuous curve. From time to time, automobile stylists request different shapes for the various windows of automobiles and these shapes are sometimes very simple and sometimes very complicated.
Simple shapes are easy to accomplish by the gravity sag method by supporting one or more glass sheets over a ring-type bending mold of metal which conforms in elevation and plan outline to the shape desired near the marginal periphery of the glass. Since glass has a different heat transfer rate compared to metal used in outline or ring-type molds, the glass sheet heats and cools more rapidly than the mold. Consequently, if the marginal edge of the glass extends beyond the outline of the mold, and the glass is heated and cooled sufficiently rapidly so that the difference in heat capacity between the glass and the mold is too great to enable the glass and the mold to come to an equilibrium temperature during a rapid bending and cooling operation desired for mass production purposes, the shaped glass sheet develops a stress pattern at its marginal edge that comprises a compression stress at its marginal edge portion with an accompanying tension stress inward of the margin often with its maximum along a line in the glass that is superimposed on the outline metal mold during the rapid bending and cooling operation. Such stress patterns are desirable provided the compression stress in the marginal edge portion is uninterrupted and the tension stress within the marginal edge portion is controlled. The reason for this desire is that glass is strong when stressed in compression and weak in tension. Thus, if the edge can be stressed in compression, the danger of edge breakage during handling is reduced. The weak glass zone stressed in tension is protected by the marginal edge portion stressed in compression.
When glass sheets are bent to simple curvatures wherein the glass develops a concave contour along one or both of its major dimensions, the glass sheet conforms sufficiently around its marginal edge to enable an operator to have minimum trouble in installing a curved glass window into a curved window opening in which the curved window is to be installed even though the unsupported center portion of the glass may sag during the heating operation is an uncontrolled manner. However, when glass sheets are shaped to complicated curvatures by gravity sag bending, the only way the glass sheet curvature can be controlled to form a complicated shape including a portion having concave elevation and a portion having convex elevation is by heating the supported glass to its deformation temperature and sagging the glass onto a continuous shaping surface that has previously been shaped.
Conventionally, automobile manufacturers submit patterns or dyes showing a shape and outline desired for a curved window in an automobile and a master mold of ceramic material is produced by applying to the patterns supplied by the automobile manufacturer a fence of a flexible board-like material, such as Masonite, surrounding the pattern and casting into the volume formed by the upper surface of the pattern and the Masonite fence a flowing ceramic material that hardens to form a shape complementary to the shape of the surface of the pattern. A typical ceramic material is sold under the trademark PURETAB.
A mold so formed from a castable refractory material is termed a ceramic pan mold and has a high thermal capacity. Even though it is possible to bend glass sheets to conform to the shape of a ceramic pan mold so formed, it is necessary to heat the mold and the supported glass to the glass deformation temperature at a very slow rate of heating, and, after the glass sheet has conformed to the shaping surface of the ceramic pan mold, cool the glass sheet-laden mold very slowly so as to enable both the mold and the glass to maintain a changing temperature that provides a very small temperature difference between the glass sheet and the mold to avoid glass breakage and/or warping from the mold shape. Furthermore, glass sheets shaped and cooled while supported on high heat capacity ceramic pan molds do not have the desired stress pattern that results when glass sheets are shaped by gravity sagging and cooled while supported on outline metal molds that support the shaped glass adjacent to and slightly inward of its marginal periphery.
Normally, glass sheets have been shaped by gravity sagging by heating the sheets to their deformation temperature and sagging them until they conform to the upper shaping surface of an outline or skeleton metal mold of concave elevation. While the marginal portion of glass sheets so shaped conform closely to the shape of the mold, the unsupported center portion sags with only limited control as to its shape. However, as long as the sagged central portion is in the same direction as the shape provided by the concavely shaped metal mold, the customer would accept deviations in shape that incorporate a slight bit of excessive sag. However, when glass sheets are shaped to more complicated shapes involving a shaped portion of convex elevation as well as a shaped portion of concave elevation, it becomes necessary to support the deformable glass over its entire surface on a continuous shaping surface so as to avoid sag in a concave direction where a shape of convex elevation is desired. Since the solid ceramic pan molds of the prior art were usually composed of materials of high heat capacity that required the glass bending operation to be conducted too slowly to constitute efficient use of the glass sheet bending lehrs already in existence, a more efficient manner of providing complicated bends is needed. Furthermore, a large number of bending lehrs of the type suitable for shaping glass sheets by the gravity sag method are in existence and it would constitute a waste of capital assets not to use such bending lehrs for sag bending operations when the need exists to produce parts shaped to such complicated curvatures as those defined herein.
2. Description of the Prior Art
Steel molds of outline configuration have been used for gravity sag bending and even for press bending glass sheets into desired shapes. Samples of outline bending molds used for gravity sagging are typified in U.S. Pat. Nos. 2,608,030 to Jendrisak; 3,137,558 to Oberstar; 3,155,485 to Ritenour et al; 3,230,062 to Leflet, Jr.; 3,265,488 to Ross et al; 3,278,287 to Leflet, Jr. et al and 3,281,231 to McKelvey et al.
Multiple bending of glass sheets of different compositions prior to lamination is shown in U.S. Pat. No. 3,300,351 to Richardson. Simultaneous bending of two pairs of glass sheets and then laminating selected pairs is disclosed in U.S. Pat. No. 2,314,325 to Binkert and U.S. Pat. No. 3,453,161 to Golightly. U.S. Pat. No. 2,314,325 refers to using glass spacer sheets that are bent simultaneously with a pair of glass sheets to be laminated and then discarded.
Glass sheets are also formed by press bending between molds of complemental curvature with fiber glass or other resilient layers of fiber mat material interposed between at least one of the molds between which the glass sheet is shaped and a glass surface, as disclosed in U.S. Pat. Nos. 3,329,494 to Carson et al; 3,523,783 to Clark et al; 3,514,590 to Tank; 3,634,059 to Miller; 3,682,613 to Johnson et al; 3,713,798 to Stilley et al; 3,741,743 and 3,816,089 to Seymour and 3,899,316 to Ehlers. Press bending vertically supported glass sheets requires tongs or other glass supporting elements that complicate the shaping operation. Press bending horizontally supported sheets that are redeposited on conveying mechanisms causes problems of handling sheets that are bent to shapes other than simple curvatures.
In the early days of shaping glass sheets, glass sheets were shaped by gravity sagging into conformity with a metal mold having a central recessed portion such as depicted in U.S. Pat. No. 280,143 to DeVoursney. However, the mass of such molds necessitated too long a time for the glass to be annealed without breaking after it was shaped. U.S. Pat. No. 417,097 to Scott discloses a solid gravity sag mold in which a shaping surface of plaster is coated with a charcoal facing. U.S. Pat. No. 760,959 to Connington and U.S. Pat. No. 833,436 to Borland et al show a mold having a facing surface of plaster of paris.
U.S. Pat. No. 1,519,277 to Taylor discloses heavy cast iron molds of the gravity sag type of concave elevation.
U.S. Pat. No. 2,876,594 to McRoberts and Black shows a gravity sag mold of convex elevation for sagging glass sheets onto a thin metal sheet supported on an outline mold, while U.S. Pat. No. 3,136,619 to McRoberts and Golightly shows a gravity sag mold of convex elevation having a filler of soft refractory material filling a recess enclosed by an outline mold frame that engages the undersurface of a supported glass sheet in spaced relation to a metal shaping surface that supports the soft refractory filler.
In recent years, glass sheets to be laminated into shaped windshields have been bent while supported on outline molds of steel having upper edge surfaces conforming to the shape desired slightly inward of the marginal edge of the glass. Having the shaped glass sag by gravity as a result of heating it to its softening point has provided suitable support around its periphery where the glass shape is especially critical because of the need to match the curvature of the glass with the opening in which the curved window is to be installed. However, when the glass is supported around its margin only, the unsupported central portion of the glass is likely to sag with only limited control and produce an optically distasteful product. It is also desirable in shaping glass to avoid contacting the glass or bringing it into close proximity to the shaping surface of a glass sheet supporting member having a significantly higher heating capacity than that of the glass sheet to be shaped. Using high heat capacity for the supporting member necessitates a long time to cool the glass and its supporting member without causing tension stress that induces glass breakage.